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Accelerometer measuring speed ??

Discussion in 'General Electronics Discussion' started by Divedeep, Mar 6, 2014.

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  1. Divedeep

    Divedeep

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    Feb 2, 2014
    Hi,

    Has anyone had any use of accelerometers?

    I would like to use them to calculate speed underwater.

    Reading up on them it appears it is possible to calculate speed using them. The problem is that gravity adds tiny errors into the mix as you are unable to differentiate between gravitational force a velocity.

    These tiny errors become huge errors over a short period of time and the data is pretty much useless.

    Has anyone used an accelerometer for anything similar??
    Would using multiple sensors and averaging results help at all?
     
  2. jpanhalt

    jpanhalt

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    Nov 12, 2013
  3. Divedeep

    Divedeep

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    Feb 2, 2014
    Yes i have read about INS.
     
  4. jpanhalt

    jpanhalt

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    Nov 12, 2013
    OK, so what is your question?

    In your original question, you asked whether accelerometers had been used to calculate speed. The answer is yes, for example, INS. But, it is somewhat complex to do.

    John
     
  5. Divedeep

    Divedeep

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    0
    Feb 2, 2014
    My question was has anyone used accelerometers for calculating speed. As in has anyone on this forum done this..

    My second question was, would using multiple sensors give me better results??

    I know what INS is etc. I know how it works and whats involved. I havnt had much use with accelerometers though.

    The complexity doesnt phase me im just looking for pointers.
     
  6. jpanhalt

    jpanhalt

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    Nov 12, 2013
    Sorry, can't help you. I used accelerometers only to measure acceleration and tilt relative to Earth.

    John
     
  7. Laplace

    Laplace

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    Apr 4, 2010
    What coordinate system are you using as a reference for measuring the direction of the acceleration? As the angular orientation of the underwater platform changes, how will you keep the accelerometers aligned with the coordinate system?
     
  8. Divedeep

    Divedeep

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    Feb 2, 2014
    I do not yet have anything in place. I plan to have the sensors mounted on a gimbal. This should help to keep the axis at a constant angle.
    The gimbal would only have pitch and roll. Yaw movement will be fixed.
     
  9. Laplace

    Laplace

    1,252
    184
    Apr 4, 2010
    Analog Dialogue Volume 43 Number 2, 2009, has an article titled "Accelerometers—Fantasy and Reality" By Harvey Weinberg.

    "As applications engineers supporting ADI’s compact, low-cost, gravity-sensitive iMEMS® accelerometers, we get to hear lots of creative ideas about how to employ accelerometers in useful ways, but sometimes the suggestions violate physical laws! We’ve rated some of these ideas on an informal scale, from real to dream land:
    • Real – A real application that actually works today and is currently in production.
    • Fantasy – An application that could be possible if we had much better technology.
    • Dream Land – Any practical implementation we can think of would violate physical laws.

    ...
    "Personal Navigation. In this application, position is determined by dead reckoning (double integration of acceleration over time to determine actual position).

    "Dream Land. Long-term integration results in a large error due to the accumulation of small errors in measured acceleration. Double integration compounds the errors. Without some way of resetting the actual position from time to time, huge errors result. This is analogous to building an integrator by simply putting a capacitor across an op amp. Even if an accelerometer’s accuracy could be improved by ten or one hundred times over what is currently available, huge errors would still eventually result. They would just take longer to happen. Accelerometers can be used with a GPS navigation system when the GPS signals are briefly unavailable. Short integration periods (a minute or so) can give satisfactory results, and clever algorithms can offer good accuracy using alternative approaches. When walking, for example, the body moves up and down with each step. Accelerometers can be used to make very accurate pedometers that can measure walking distance to within ±1%."
     
  10. Divedeep

    Divedeep

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    0
    Feb 2, 2014
    Hmmm. That doesnt sound promising.

    Has anyone got any ideas how speed could be measured underwater.

    I have looked at pitot tubes but these would not work for my application.

    I am trying to calculate the speed of a diver.
     
  11. jpanhalt

    jpanhalt

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    4
    Nov 12, 2013
    Can you not measure the speed relative to the water with a little propeller and motor or encoder in the same manner that wind speed is measured? It does not have to be very big.

    If you want to determine location, however, you have to consider that the water may be moving to relative to Earth. If you can estimate the average movement of the water (i.e., velocity and direction), say with a fixed sensor, then dead-reckoning can be used to estimate location.

    John
     
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